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cucumber, tomatoes, peppers, green onions

New Vegetable Gardener — Irrigation and Fertilizer (Getting a Little Technical)

This topic is very critical to the productivity of your new vegetable garden. Water and fertilizer are closely linked. Without water, nutrients (fertilizer) will not be made available to your plants. Without nutrients, water, and sunlight, plants cannot make food for themselves.

Our sandy soils have insufficient nutrients to produce a quality vegetable. The use of organic matter (compost) is helpful to build structure in the soil but is not a sufficient source of nutrients to produce a quality vegetable. Water and nutrients are closely tied together. Keeping the garden soil moderately moist throughout the gardening season will ensure that when a plant needs a nutrient, it can get it (if sufficient nutrients have been made available).

The Florida Vegetable Gardening Guide suggests that you apply 2-3 pounds of a vegetable fertilizer to every 100 square feet of the vegetable garden. Using a synthetic fertilizer with an analysis of 10-10-10 (10% Nitrogen, 10% Phosphorous, 10% (K) potassium) with other secondary and minor elements is recommended. However, if you wish to use an organic fertilizer instead, that will be perfectly fine. Your vegetable plants do not care which type of fertilizer you use. The fertilizer should be a complete fertilizer (shows numbers for N-P-K) with all the nutrients the vegetable plant needs to grow.

Synthetic fertilizers are already in an inorganic form so plants can use it almost immediately. Organic fertilizers need a little while to be converted to an inorganic form by soil microbes before the plants can use them. So add organic fertilizers a few weeks before you add your plants or seeds to the garden. Once your transplants and seeds are planted, you should add a small amount of fertilizer (about ¼ pound per 10 foot row) every three weeks throughout the gardening season.

The reason you want a water source near the garden is so you can maintain a moderately moist soil throughout the season. When you water, water early in the day and make a determined effort to keep water off the leaves of your vegetable plants to reduce the incidence of diseases. So, an overhead sprinkler system or your yard irrigation system would not be good choices to water the vegetable garden. Additionally, the Water Management District restricts the number of days you can water per week if you water with an in ground irrigation system. That may not be a sufficient number of days to keep your garden soil consistently moist throughout the gardening season.

There are four irrigation methods that are exempt from the Water Management District rules:

  1. Water with a garden hose. Use a hose with a shut off valve on the end that the water runs out. Add a three foot wand and a water breaker. The shut off valve keeps the water from running out the end of the hose when the water is turned on or off at the spigot. The wand keeps you from bending over to water at the base of the plants and reduces the amount of water that gets on the leaves. The water breaker produces a soft shower effect and is used to water the soil around your plants without creating soil splashing on the leaves and stems.
  2. Soaker hose. Make sure you have the blue washer inserted into the end that attaches to the spigot so water pressure will be more uniform along the length of the hose.
  3. Micro-jet irrigation. Is a low-volume (gallons per hour of water) irrigation system using small spray nozzles on a plastic stake that can be pulled out of the soil or pushed into the soil as necessary to keep the spray pattern within the garden bed.
  4. Micro-irrigation. Is a low-volume (gallons per hour of water), low-flow (drip) irrigation system. Water is sent through a water delivery line and exits through an emitter that is engineered to apply small quantities of water into the soil near the plants roots.

How much water is needed for vegetable plants to be productive? For the first several weeks, an amount of water equal to one inch per square foot (~.624 gallons of water) per week is adequate. Once flowers of fruit appear on the vegetable plant, an amount of water equal to two inches per square foot per week is needed.

What does that mean in real numbers? If you have a garden bed 4’x16’ (64 square feet), in the beginning you should apply approximately 40 gallons of water per week to that bed (64 x .624 = 39.936). Then you would double this amount once flowers or fruit appear.

If you are using a micro-irrigation system, with a battery powered timer, and wondering how long you should set the timer to run to provide sufficient irrigation to the 64 square foot garden bed, you would need to know the rate at which the water is emitted through the drip emitter. One manufacturer has a hose that has a drip emitter every twelve inches and emits water at the rate of .219 gallons of water per hour. To provide 40 gallons of water (1” per square foot) to the 64 square foot garden bed, there would be four drip emitter hoses with 1-foot spacing between them  (64 emitters in the garden bed). In one hour, those 64 emitters would be delivering 14.016 gallons of water (64 x .219 = 14.016). To deliver 40 gallons of water per week to the 64 square foot garden bed would take approximately 2.85 hours (40 / 14.016 = 2.85) or 171 minutes. To more uniformly apply this 40 gallons of water per week, you would set up your drip irrigation timer to water 57 minutes (171 / 3 = 57) for three days (M, W, F) and you would have a more consistently moist garden soil.

Water and fertilizer are closely linked. Your goal is to keep the water and the fertilizer in the root zone; compost will help. It is the soil water combination that carries fertilizer into the plant through the roots. Do not over water as this causes leaching and loss of fertilizers (wastes money). Do not use more fertilizer than needed as this can lead to increased surface and groundwater contamination.